Rocket Innovations: Buoyant Spacecraft

FMX_ascender 600x338The fundamental kinematic characteristic of orbiting spacecraft is high-velocity ballistic trajectory, which can only be achieved at very high altitude where drag forces drop to near zero. Thus, the problem of launch to orbit can be decomposed into two separate issues: altitude and velocity. Rockets solve this problem all at once, using reaction force to accelerate very rapidly to high-altitude ballistic flight. Rockets must accelerate at high levels compared to gravity. Such rapid acceleration requires the rapid release of enormous amounts of energy, which is inherently dangerous. In addition, rockets are highly complex vehicles, prone to failure, and expensive to use only once and then throw away. Unfortunately, rockets are the only mature technology we have for getting into space.

Altitude has been approached via other means. Rockets can be launched from a conventional horizontal-takeoff aircraft. The aircraft uses aerodynamic lift to gain some initial altitude before launching the rocket to orbit. By launching at altitude, the rocket can expend less propellant and need not push as massive a column of air out of its way. The same impulse will propel the rocket much further in the rarified low-pressure atmosphere than from the ground.

Another means of achieving altitude is atmospheric buoyancy. Buoyancy can be used to boost a vehicle into the upper atmosphere and support it at a very high altitude. Balloons have the capability to go much higher than aircraft. Balloons have been used as a first stage to raise rockets for launch from the stratosphere. The same advantages that apply to air-launch are significantly improved with balloon launch: less propellant, less mass to boost, and less atmospheric drag.

Combining all three forces, buoyancy, aerodynamic lift, and reaction force, together into a single vehicle is a key innovation that opens up a novel approach to attaining low Earth orbit. Such a buoyant spacecraft uses gravity and the atmosphere as assets, rather than obstacles to overcome, in order to fly to space. Because it floats and is supported by the atmosphere, the buoyant spacecraft need not initially follow a ballistic trajectory. Therefore, it may accelerate more slowly, allowing the use of high-efficiency, high specific impulse engines such as VASIMR or other electrical propulsion. The spacecraft achieves orbital velocity over time rather than all at once.

There are numerous benefits to buoyant spacecraft design. The buoyant spacecraft is extremely safe. By using high-efficiency electric propulsion, there is no need for highly reactive chemical fuels, so there is little danger of explosion. Where engine failure in a rocket is a catastrophic emergency that must be handled within seconds, the buoyant spacecraft can continue to glide forward, stable and safe. The emergency is downgraded to a problem that the crew can take hours or even days to evaluate and repair. Since no booster rockets are used, there is no danger to habitable areas below the flight path from lower stages dropped from the vehicle. The spacecraft is also relatively unaffected by impacts from meteors or orbital debris. The skin of the vehicle can be made of high strength Kevlar-like materials to resist impacts from micrometeoroids. If punctured by a larger object, the craft will not “pop” like a balloon because the lifting gas is not under a high pressure. Instead, a puncture would cause a slow leak. Redundancies in the lifting cells allow the lifting gas to be shifted away from the punctured cell to a different cell and rebalanced for continued flight. Punctures like this become a maintenance issue rather than a failure mode.

Unlike expendable rockets that are flown once and then discarded, the buoyant spacecraft is highly reusable. Because the launch to orbit is relatively gentle and the reentry is benign, damage to the spacecraft from a given flight is minimal. After a return from one flight, the vehicle can be turned around quickly, needing mainly refueling and restocking. Over sustained usage, some ongoing maintenance will be required (e.g. repairing punctures). Consequently the spacecraft can provide reliable operation over hundreds of flights. The cost of the vehicle can be amortized over many flights, drastically lowering the cost per kilogram of getting to orbit.

If feasible, the benefits of this approach are quite attractive. Although the concept of buoyant spacecraft has been proposed elsewhere, the aerodynamics of very large, buoyant spacecraft flying at hypersonic velocities at the edge of the atmosphere and out to space is a relatively unexplored domain.  Unfortunately, not much information is available in the literature about the aerodynamics or feasibility of this approach.  Consequently, this continues to be an area ripe for innovation and experimentation.